Process of making pins for connecting carbon electrodes

ABSTRACT

A process for preparing pins for connecting carbon electrodes is presented. In particular, a process for preparing pins for connecting carbon electrodes including the steps of combining calcined coke, a liquid pitch binder and carbon fibers derived from mesophase pitch to form a pinstock blend; extruding the pinstock blend to form a green pinstock; baking the green pinstock to form a carbonized pinstock; and graphitizing the carbonized pinstock by maintaining the carbonized pinstock at a temperature of at least about 2500° C. for no more than about 18 hours is presented. The pins prepared by the inventive process are also presented.

TECHNICAL FIELD

The present invention relates to a pin for connecting carbon electrodes,and a process for preparing the inventive pin. More particularly, theinvention concerns a pin for connecting carbon electrodes, such asgraphite electrodes, formed by processing a blend of calcined coke,pitch and carbon fibers derived from mesophase pitch.

BACKGROUND ART

Carbon electrodes, especially graphite electrodes, are used in the steelindustry to melt the metals and other ingredients used to form steel inelectrothermal furnaces. The heat needed to melt metals is generated bypassing current through a plurality of electrodes, usually three, andforming an arc between the electrodes and the metal. Currents in excessof 100,000 amperes are often used. The resulting high temperature meltsthe metals and other ingredients. Generally, the electrodes used insteel furnaces each consist of electrode columns, that is, a series ofindividual electrodes joined to form a single column. In this way, aselectrodes are depleted during the thermal process, replacementelectrodes can be joined to the column to maintain the length of thecolumn extending into the furnace.

Generally, electrodes arc joined into columns via a pin (sometimesreferred to as a nipple) that functions to join the ends of adjoiningelectrodes. Typically, the pin takes the form of opposed male threadedsections, with at least one end of the electrodes comprising femalethreaded sections capable of mating with the male threaded section ofthe pin. Thus, when each of the opposing male threaded sections of a pinare threaded into female threaded sections in the ends of twoelectrodes, those electrodes become joined into an electrode column.Commonly, the joined ends of the adjoining electrodes, and the pintherebetween, are referred to in the art as a joint.

Given the extreme thermal stress that the joint (and indeed theelectrode column as a whole) undergoes, mechanical factors such asthermal expansion must be carefully balanced to avoid damage ordestruction of the electrode column or individual electrodes. Forinstance, longitudinal (i.e., along the length of thepin/electrode/electrode column) thermal expansion of the pin, especiallyat a greater rate than that of the electrodes, can force the jointapart, reducing effectiveness of the electrode column. A certain amountof transverse (i.e., across the diameter of the pin/electrode/electrodecolumn) thermal expansion of the pin in excess of that of the electrodesmay be desirable to form a firm connection between pin and electrode;however, if the transverse thermal expansion of the pin greatly exceedsthat of the electrode, damage to the electrode may result, in the formof cracking or splitting. Again, this can result in reducedeffectiveness of the electrode column, or even destruction of the columnif the damage is so severe that a joint fails. Thus, control of thethermal expansion of a pin, in both the longitudinal and transversedirections, is of paramount importance.

There have been references to the use of mesophase pitch-based carbonfibers to improve specific properties of bulk graphite products such aselectrodes. For instance, Singer, in U.S. Pat. No. 4,005,183, describesthe production of mesophase pitch-based fibers and states that, becauseof their low electrical resistivity, these fibers can be employed asfiller material in the production of graphite electrodes. In BritishPatent 1,526,809 to Lewis and Singer, 50% to 80% by weight of carbonfibers are added to 20% to 50% by weight of pitch binder and thenextruded to form a carbon artifact that can be graphitized. Theresulting article exhibits relatively low longitudinal thermalexpansion.

In U.S. Pat. No. 4,998,709, Griffin et al. attempt to address theproblems caused by excessive longitudinal thermal expansion of electrodepins by preparing a graphite nipple (i.e., pin) with mesophasepitch-based carbon fibers included in the extrusion blend. The carbonfibers used by Griffin et al. have a Young's modulus of greater than55×10⁶pounds per square inch (psi), and are present in the blend atabout 8 to 20 weight percent. The blend is extruded, baked, and thengraphitized for from about 5 to 14 days to produce the nipple. Althoughnipples produced by the Griffin et al. process show a decrease in thecoefficient of thermal expansion (CTE) in the longitudinal direction,they also show an undesirable increase in CTE in the transversedirection, an increase in electrical resistivity and a decrease in themodulus of rupture. In addition, the graphitizing time is extremely longcompared with times that would be advantageous for commercialproduction.

What is desired, therefore, is a pin for connecting carbon electrodes,the pin having reduced CTE in the longitudinal direction as comparedwith art-conventional pins, without sacrificing transverse CTE orresistivity and modulus of rupture. Especially desirable is such a pinthat is prepared by a process that does not require 5 days ofgraphitization. It is also highly desirable to achieve these propertybenefits without using high quantities of expensive materials.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a process forpreparing pins for connecting carbon electrodes.

It is another object of the present invention to provide a process forpreparing pins for connecting carbon electrodes, the pins having reducedlongitudinal coefficient of thermal expansion as compared toart-conventional pins.

It is yet another object of the present invention to provide a processfor preparing pins for connecting carbon electrodes, the pins havingreduced longitudinal coefficient of thermal expansion as compared toart-conventional pins, without substantial sacrifice of transverse CTEor resistivity while also increasing the modulus of rupture.

It is still another object of the present invention to provide a processfor preparing pins for connecting carbon electrodes, the pins havingreduced longitudinal coefficient of thermal expansion as compared toart-conventional pins, wherein the process requires graphitization timessignificantly shorter than 5 days.

These objects and others that will become apparent to the artisan uponreview of the following description can be accomplished by providing aprocess for preparing pins for connecting carbon electrodes, the processincluding combining calcined coke, a liquid pitch binder and carbonfibers derived from mesophase pitch to form a pinstock blend; extrudingthe pinstock blend to form a green pinstock; baking the green pinstockto form a carbonized pinstock; and graphitizing the carbonized pinstockby heating to a temperature of at least about 2500° C. and maintainingit at that temperature for no more than about 18 hours.

In the inventive process, the carbon fibers are preferably present at alevel of about 0.5 to about 5 parts by weight of carbon fibers per 100parts by weight of calcined coke, or at about 0.4% to about 4.0% byweight of the total mix components, have a Young's modulus aftergraphitization of no more than about 40×10⁶ psi, an average diameter ofabout 6 to about 15 microns, and a length of about ⅙ inch to about 1inch. Most advantageously, the carbon fibers are added to the pinstockblend as bundles, each bundle containing from about 2000 to about 20,000fibers. The baking of the green pinstock preferably takes place at atemperature of up to about 700 to about 1000°C. in a non-oxidizing orreducing environment, and graphitization is more preferably at atemperature of from about 2500 to about 3400° C.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

As noted above, pins for connecting graphite electrodes can befabricated by first combining calcined coke, pitch and mesophasepitch-based carbon fibers into a pinstock blend. More specifically,crushed, sized and milled calcined petroleum coke is mixed with acoal-tar pitch binder to form the blend. The particle size of thecalcined coke is selected according to the end use of the electrode, andis within the skill in the art. Generally, in graphite electrodes foruse in processing steel, particles up to about 25 millimeters (mm) inaverage diameter arc employed in the blend. Other ingredients that maybe incorporated into the blend at low levels include iron oxides toinhibit puffing (caused by release of sulfur from its bond with carboninside the coke particles) and oils or other lubricants to facilitateextrusion of the blend.

Also included in the blend are mesophase pitch-based carbon fibers. Suchfibers are produced from pitch that has been at least partiallytransformed to a liquid crystal, or so-called mesophase, state. Thefibers used should advantageously have a Young's modulus (aftercarbonization) of about 15×10⁶ psi to about 40×10⁶ psi. They preferablyhave an average diameter of about 6 to about 15 microns, a tensilestrength of about 200×10³ psi to about 400×10³ psi, and are about ⅙ inchto about 1 inch in length on average. Most advantageously, the fibersare added to the blend as bundles containing between about 2000 andabout 20,000 fibers per bundle, compacted with the use of a sizing.

As noted, the carbon fibers to be included in the blend are based onmesophase pitch. Mesophase pitch can be prepared from feedstocks such asheavy aromatic petroleum streams, ethylene cracker tars, coalderivatives, petroleum thermal tars, fluid cracker residues and pressuretreated aromatic distillates having a boiling range from 340° C. toabout 525° C. The production of mesophase pitch is described in, forexample, U.S. Pat. No. 4,017,327 to Lewis et al., the disclosure ofwhich is incorporated herein by reference. Typically, mesophase pitch isformed by heating the feedstock in a chemically inert atmosphere (suchas nitrogen, argon, xenon, helium or the like) to a temperature of about350° C. to 500° C. A chemically inert gas can be bubbled through thefeedstock during heating to facilitate the formation of mesophase pitch.For preparation of carbon fibers, the mesophase pitch should have asoftening point, that is, the point at which the mesophase pitch beginsto deform, of less than 400° C. and usually less than about 350° C. Ifthe pitch has a higher softening point, formation of carbon fibershaving the desired physical properties is difficult.

Once the mesophase pitch is prepared, it is spun into filaments of thedesired diameter, by known processes such as by melt spinning,centrifugal spinning, blow spinning or other processes which will befamiliar to the skilled artisan. Spinning produces carbon fiberssuitable for use in preparing the pin of the present invention. Thefilaments arc then thermoset at a temperature no higher than thesoftening point of the pitch (but usually above 250° C.) for about 5 to60 minutes, then further treated at extremely high temperatures, on theorder of up to about 1000° C. and higher, and in some cases as high asabout 3000° C. more typically about 1500° C. to 1700° C., to carbonizethe fibers. The carbonization process takes place in an inertatmosphere, such as argon gas, for at least about 0.5 minutes. Mostcommonly, carbonization uses residence times of between about 1 and 25minutes. The fibers are then cut to length and formed into bundles. Suchfibers, bundled as described, are commercially available from BP/AmocoCompany of Alpharetta, Ga. and Mitsubishi Chemical Company of Tokyo,Japan.

The carbon fibers are preferably included in the blend at a level ofabout 0.5 to about 6 parts by weight of carbon fibers per 100 parts byweight of calcined coke. Most preferably, the fibers are present at alevel of about 1.25 to about 5 parts by weight fibers per 100 parts byweight of coke. In terms of the blend as a whole, the carbon fibers areincorporated at a level of about 1% to less than about 5% by weight.

After the blend of calcined coke, pitch binder, carbon fibers, etc. isprepared, the pin body is formed (or shaped) by extrusion though a dieor molded in conventional forming molds to form what is referred to as agreen pinstock. The forming, whether through extrusion or molding, isconducted at a temperature close to the softening point of the pitch,usually about 100° C. or higher. Although the die or mold can form thepin in substantially final form and size, machining of the finished pinis usually needed, at the very least to provide threads which may berequired. As would be apparent, the pins are sized so as to have adiameter no greater than that of the electrodes to be joined into anelectrode column. Typically, the pins have a diameter that is about 30%to about 60% of the diameter of the electrode. Thus, for electrodeswhose diameter can vary between about 15 and 30 inches, the pins have adiameter of about 4.5 to about 18 inches.

After extrusion, the green pinstock is heat treated by baking at atemperature of between about 700° C. and about 1100° C., more preferablybetween about 800° C. and about 1000° C., to carbonize the pitch binderto solid coke, to give the pin permanency of form, high mechanicalstrength, good thermal conductivity, and comparatively low electricalresistance. The green pinstock is baked in the relative absence of airto avoid oxidation. Baking should be carried out at a rate of about 1°C. to about 5° C. an hour to the final temperature. After baking, thepin may be impregnated one or more times with coal tar or petroleumpitch, or other types of pitches known in the industry, to depositadditional pitch coke in any open pores of the pin. Each impregnation isthen followed by an additional baking step.

After baking, the pin, referred to at this stage as carbonized pinstock,is then graphitized. Graphitization is by heat treatment at a finaltemperature of between about 2500° C. to about 3400° C. for a timesufficient to cause the carbon atoms in the calcined coke and pitch cokebinder to transform from a poorly ordered state into the crystallinestructure of graphite. Advantageously, graphitization is performed bymaintaining the carbonized pinstock at a temperature of at least about2700° C., and more advantageously at a temperature of between about2700° C. and about 3200° C. At these high temperatures, elements otherthan carbon are volatilized and escape as vapors. The time required formaintenance at the graphitization temperature using the process of thepresent invention is no more than about 18 hours, indeed, no more thanabout 12 hours. Preferably, graphitization is for about 1.5 to about 8hours.

As noted, once graphitization is completed, the finished pin can be cutto size and then machined or otherwise formed into its finalconfiguration. Typically, the pin is tapered from the middle to eitherend, and then threads are machined into either end of the pin, to permitmating with corresponding threads in the ends of carbon electrodes, toform the electrode column. Given its nature, the graphite pin permitsmachining to a high degree of tolerance, thus permitting a strongconnection between pin and electrode.

The pins prepared in accordance with the present invention exhibit asubstantial reduction in longitudinal CTE as compared with pins preparedwithout carbon fibers. The pins show an increase in flexural strength(i.e., modulus of rupture) and an increase in Young's modulus, without aconcomitant significant increase in transverse CTE or specificresistance, without the requirement of commercially disadvantageousgraphitization times.

The following examples are presented to further illustrate and explainthe present invention and should not be viewed as limiting in anyregard. Unless otherwise indicated, all parts and percentages are byweight, and are based on the weight of the product at the particularstage in processing indicated.

EXAMPLE I

A trial was conducted using bundles of mesophase pitch-based carbonfiber designated as Grade K 223-SE obtained from Mitsubishi ChemicalCompany of Tokyo, Japan. The fibers had an average diameter of 10microns, a Young's modulus of 30×10⁶ psi and an average tensile strengthof 340×10³ psi. The fibers were compacted into bundles of approximately12,000 fibers with a sizing, and chopped into ¼ inch lengths.

The fiber bundles were blended in a cylinder mixer with calcined cokeparticles and coal tar binder pitch. The blend contained 4.5 parts byweight of carbon fibers per 100 parts by weight of calcined coke per 32parts by weight of pitch. About 1 part by weight of iron oxide was addedto inhibit coke puffing and about 1 part by weight of oil was added asan extrusion aid. The weight percentage of fibers in the total blend was3.2%.

The ingredients were blended for about 1 hour while heating up to about160° C., and then extruded at about 110° C. into a 317 mm diameterpinstock. The pinstock was baked to 800° C. at a rate of 2° C./hour inan atmosphere protected from air, and then impregnated with pitch andrebaked to 800° C., followed by a second impregnation and rebake. Thecarbonized pinstock was then heated to about 2700° C. in agraphitization furnace and maintained above about 2700° C. for about 5hours.

As a control, pinstock was formed in the same manner with the sameingredients and processing parameters, but without the addition of anycarbon fibers.

The graphite properties obtained for the pinstock with and withoutfibers are summarized in Table I.

TABLE I Pinstock Pinstock With Fibers (3.2%) Without Fibers LongitudinalCTE/° C. −1.8 × 10⁻⁷  0.8 × 10⁻⁷ (from 30-110° C.) Transverse CTE/° C.18.9 × 10⁻⁷ 18.8 × 10⁻⁷ (from 30-110° C.) Flexural Strength (MPa) 24.922.3 Young's Modulus (GPa) 19.6 16.9 Specific Resistance (micro-ohm-m)3.74 3.67 Density (g/cc) 1.624 1.650

As can be seen, the addition of 3.2% by weight carbon fibers led to asubstantial reduction in longitudinal CTE and an increase in flexuralstrength with no significant adverse effect on transverse CTE orspecific resistance.

EXAMPLE II

A trial was conducted using bundles of mesophase pitch-based carbonfiber designated as Thornel Carbon Fibers, Grade GPX obtained fromBP/Amoco Company of Alpharetta, Ga. The fibers had an average diameterof 11 microns, a Young's modulus of 23×10⁶ psi and an average tensilestrength of 212×10³ psi. The fibers were compacted into bundles ofapproximately 4000 fibers with a sizing, and chopped into ¼ inchlengths.

The fiber bundles were blended in a cylinder mixer with calcined cokeparticles and coal tar binder pitch. The blend contained 2 parts byweight of carbon fibers per 100 parts by weight of calcined coke per 32parts by weight of pitch. About 1 part by weight of iron oxide was addedto inhibit coke puffing and about 1 part by weight of oil was added asan extrusion aid. The weight percentage of fibers in the total blend was1.5%.

The blend was blended for about 1 hour while heating up to about 160° C.and then extruded at about 110° C. into a 150 mm diameter pinstock. Thepinstock was baked to 800° C. at a rate of 2° C./hour in an atmosphereprotected from air, and then impregnated with pitch and rebaked to 800°C. followed by a second impregnation and rebake. The carbonized pinstockwas then heated to about 3000° C. in a graphitization furnace andmaintained at about 3000° C. for about 1 hour.

As a control, pinstock was formed in the same manner with the sameingredients and processing parameters, but without the addition of anycarbon fibers.

The graphite properties obtained for the pinstock with and withoutfibers are summarized in Table II.

TABLE II Pinstock With Fibers (1.5%) Control Levels PinstockLongitudinal CTE/° C. −0.5 × 10⁻⁷  0.9 × 10⁻⁷ (from 30-110° C.)Transverse CTE/° C. 17.2 × 10⁻⁷ 17.7 × 10⁻⁷ (from 30-110° C.) FlexuralStrength (psi) 3369 3011 Young's Modulus (psi) 2.47 2.31 SpecificResistance (micro-ohm-m) 4.88 4.89 Density (g/cc) 1.76 1.78

It is evident that even carbon fiber levels of 1.5% by weight result inlower longitudinal and transverse CTE and increased strength of thepinstock without significantly affecting other properties.

The above description is intended to enable the person skilled in theart to practice the invention. It is not intended to detail all of thepossible variations and modifications that will become apparent to theskilled worker upon reading the description. It is intended, however,that all such modifications and variations be included within the scopeof the invention that is defined by the following claims. The claims areintended to cover the indicated elements and steps in any arrangement orsequence that is effective to meet the objectives intended for theinvention, unless the context specifically indicates the contrary.

What is claimed is:
 1. A process for preparing pins for connecting carbon electrodes, the process comprising (a) combining calcined coke, a liquid pitch binder and carbon fibers derived from mesophase pitch to form a pinstock blend; (b) extruding the pinstock blend to form a green pinstock; (c) baking the green pinstock to form a carbonized pinstock; and (d) graphitizing the carbonized pinstock by maintaining the carbonized pinstock at a temperature of at least about 2500° C. for no more than about 18 hours, to form a pin for connecting carbon electrodes.
 2. The process of claim 1 herein the carbon fibers are present at a level of about 0.5 to about 5 parts by weight of carbon fibers per 100 parts by weight of calcined coke.
 3. The process of claim 2 wherein the carbon fibers have a Young's modulus of no more than about 40×10⁶ psi.
 4. The process of claim 3 wherein the carbon fibers have a Young's modulus of about 15×10⁶ psi to about 40×10⁶ psi.
 5. The process of claim 4 wherein the carbon fibers have an average diameter of about 6 to about 15 microns.
 6. The process of claim 5 wherein the carbon fibers have a length of about ⅙ inch to about 1 inch.
 7. The process of claim 1 wherein the carbon fibers are added to the pinstock blend as bundles, each bundle containing from about 2000 to about 20,000 fibers.
 8. The process of claim 1 wherein the green pinstock is baked at a temperature of about 700 to about 1100° C. in an inert environment.
 9. The process of claim 8 wherein the carbonized pinstock is graphitized by maintaining it at a temperature of from about 2500 to about 3400° C. for from about 1.5 hours to about 8 hours.
 10. A process for preparing pins for connecting carbon electrodes, the process comprising (a) combining calcined coke, a liquid pitch binder and from about 0.5 to about 5 parts by weight carbon fibers derived from mesophase pitch per 100 parts by weight of calcined coke to form a pinstock blend, the carbon fibers having a Young's modulus of no greater than about 40×10⁶ psi; (b) extruding the pinstock blend to form a green pinstock; (c) baking the green pinstock to form a carbonized pinstock; and (d) graphitizing the carbonized pinstock, to form a pin for connecting carbon electrodes.
 11. The process of claim 10 wherein the carbon fibers have a Young's modulus of about 15×10⁶ psi to about 40×10⁶ psi.
 12. The process of claim 11 wherein the carbon fibers have an average diameter of about 6 to about 15 microns.
 13. The process of claim 12 wherein the carbon fibers have a length of about ⅙ inch to about 1 inch.
 14. The process of claim 13 wherein the carbon fibers are added to the pinstock blend as bundles, each bundle containing from about 2000 to about 20,000 fibers.
 15. The process of claim 10 wherein the green pinstock is baked at a temperature of about 700 to about 1100° C. in an inert environment.
 16. The process of claim 15 wherein the carbonized pinstock is graphitized by maintaining it at a temperature of from about 2500 to about 3400° C. for from about 1.5 hours to about 8 hours. 